Earthquakes occur when vast plates, or rocks, within the Earth suddenly break or shift under stress, sending shock waves rippling.
Sudden movement along the fault causes the ground to shake.
Most earthquakes occur along fractures in the Earth's crust called faults.
Intraplate quakes occur far from plate edges and happen when stress builds up and the Earth's crust is stretched or squeezed together until it rips.
There are several different types of faults. Each can be a few inches or many hundreds of miles long. They can be horizontal, vertical, or at an angle.
Earthquake waves are measured on sensitive instruments called seismographs.
The Richter scale assigns quakes a number based on the power of its seismic waves.
Thousands of quakes occur every day around the globe, most of them too weak to be felt.
Every year about 10,000 people, on average, die as a result of earthquakes.
Volcanoes are vents in the Earth's surface from which molten rock, debris, and steam issue.
About 1,900 volcanoes are active today or known to have been active in historical times.
Almost 90 percent of volcanoes are in the Ring of Fire, a band of volcanoes circling the edges of the Pacific Ocean.
Most volcanoes occur at plate boundaries, areas where huge slabs of rock meet in the Earth's lithosphere, or outer shell.
Volcanoes can rise in subduction zones, areas where plates meet and one is pushed beneath another. Molten rock rises to the surface and forms a volcano.
Volcanoes can also arise in spreading centers, or rifts, where plates move away from each other, spreading or splitting the Earth's crust.
Intraplate volcanoes are caused by hot spots deep within the Earth. Magma rises and erupts as lava through cracks in the Earth's surface, forming volcanoes.
An eruption begins when magma, the molten rock from deep in the Earth's crust, rises toward the surface.
Volcanoes can erupt in a combination of ways: explosively with hard pyroclastic material; explosively with fluid lava (lava fountains); effusively with hard pyroclastic flows (clouds of ash and gases); and effusively with fluid lava.
Although some volcanoes are considered extinct, almost any volcano is capable of rumbling to life again.
Volcanoes provide valuable mineral deposits, fertile soils, and geothermal energy.
A tornado is a violently rotating column of air that descends from a thunderstorm to the ground.
Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120.
Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but most tornadoes occur in the United States.
Tornadoes form when warm, humid air collides with cold, dry air.
The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation.
Most tornadoes in the United States occur in Tornado Alley, a swath that stretches from Texas to Nebraska and also includes Colorado, Iowa, Indiana, Illinois, Kansas, Missouri, and Arkansas.
Tornado season begins in early spring for the states along the Gulf of Mexico. The season follows the jet stream—as it swings farther north, so does tornado activity.
Twisters are usually accompanied or preceded by severe thunderstorms, high winds, or hail.
Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours.
The average twister is about 660 feet (200 meters) wide and moves about 30 miles (50 kilometers) per hour.
Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.
A hurricane is a rotating tropical storm with winds of at least 74 miles (119 kilometers) an hour.
These storms are called hurricanes when they develop over the Atlantic or eastern Pacific Oceans.
They are called cyclones when they form over the Bay of Bengal and the northern Indian Ocean.
They are called typhoons when they develop in the western Pacific.
Most Atlantic Ocean hurricanes form near the Cape Verde Islands off Africa's west coast.
Once a tropical storm's winds hit a constant speed of at least 74 miles (119 kilometers) an hour, it becomes a hurricane.
The eye is the low-pressure center of the hurricane. Air sinks inside the eye, clearing the skies and making it relatively calm.
A ring-shaped eye wall surrounds the eye and carries the storm's most violent winds and its most intense rains.
Hurricane season in the Atlantic, Caribbean, Gulf of Mexico, and central Pacific is from June 1 to November 30. In the eastern Pacific, it is from May 15 to November 30.
Hurricanes can cause floods, flash floods, tornadoes, and landslides.
Storm surge, an abnormal rise in sea level, is usually the most dangerous part of a hurricane. Surges can cause beach erosion, wash out roads, and decimate homes.
Forecasters at the U.S. National Hurricane Center in Florida use satellite imagery, airborne reconnaissance, and computer-model projections to track storms.
